This invention relates to an improved process for manufacturing a lubricating grease in a continuous operation using a screw process unit.
A wide variety of greases have been developed over the years comprising a number of different formulations and more important a wide variation in associated properties. The main ingredient found in greases is the thickening agent or gellant and differences in grease formulations have often involved this ingredient. Typical thickening agents or gellants used in lubricating greases include the alkali metal soaps, clays, polymers, asbestos, carbon black, silica gels and aluminum complexes. Soap thickened greases constitute the largest segment by far of the commercially available greases. Simple soap greases which are salts of long chain fatty acids and a neutralizing agent are probably the most predominant type of grease in use today with lithium 12-hydroxystearate being the thickener most often used. Complex soap greases which generally comprise metal salts of a mixture of organic acids have also come into widespread use, particularly because of the various property advantages such type greases can possess. In order to satisfy the many needs of the industry, mixed base greases consisting of mixtures of soaps of two different metals including complex soaps and mixtures containing one or more non-soap gellants have been developed. Such combinations have often led to the so-called "multi-purpose" greases which are generally defined as a grease that will perform satisfactorily in two or more applications normally requiring two or more different greases. Such multi-purpose greases are in wide use and more than 90% of the grease requirements of almost any operation can be satisfied with a single grease of such type.
Besides, the thickener or gellant, other properties and characteristics of a grease can be influenced by the particular lubricating base oil and the various additives that can be used.
In addition to the individual components which make up a grease formulation, another contributing factor to the final properties and characteristics of the grease is the particular process and the conditions under which it is manufactured. Thus, process conditions such as dispersing and mixing of materials and temperature variations can be significant factors effecting the resulting grease structure, e.g., the nature of soap crystallites and fibers formed.
Since there are a variety of different greases with varying formulations and properties and since such properties can be altered, sometimes significantly, by changes in process conditions and apparatus, a great deal of flexibility is needed in the process equipment for manufacturing greases. Because of the desired flexibility and because many greases are specialty type greases made in small amounts, most grease manufacturing has been of the batch type. Batch processing generally involves the use of large kettles, typically from 1,000 to 3,000 gallons or 10,000 to 30,000 pounds capacity, with paddle agitation. Heat transfer and mixing are generally poor and thus only one batch can normally be produced in a regular 8-hour shift and some types of grease require two or three shifts for completion.
Another type of batch processor sometimes used is a Stratco contractor which has a different internal mixing configuration. In this equipment, the material is circulated by an impeller located at the bottom of the vessel. Although the heat transfer characteristics are better than in the earlier described system, production time is still several hours and not all types of greases can be made in this equipment.
There have been several attempts to develop continuous or semi-continuous grease making technology. Probably the most successful of these approaches has been Texaco's continuous grease manufacturing process. The process involves metering starting materials into a pipe which carries the mass through a reactor section, dehydration section and a finishing section in a continuous manner with recycling of the grease mixture through a shear valve during the soap conditioning step. (See. U.S. Pat. Nos. 3,475,335 and 3,475,337 as well as NLGI Spokesman, XLIV, pp. 133-136, July 1980 and NLGI Spokesman XXXII, pp. 368-373, January 1969). While greases produced by this technique are reported to be equivalent in quality to kettle-manufactured greases, it cannot be used for many complex soap greases since there is no provision for downstream addition of secondary thickening components.
In addition to the above techniques, many other variations in the processing of grease and the equipment used in such processes have been tried as disclosed for example in "Modern Lubricating Greases" by C. J. Boner, Chapter 7, pages 7.1 to 7.20, 1976.
Despite the long history of grease formulations and the manufacture thereof, there still is the need for a continuous lubricating grease manufacturing method which will provide process, quality control, and economic advantages and most important will have the flexibility to be applicable for all grease types without significant changes in equipment being used.